Recording and reproduction of sound



June 28, 1938. w KELLQGG 2,122,207

RECORDING AND REPRODUCTION OF SOUND Filed May 24, 1935 ury-0V ATTORNEY Patented June 28, 1938 UNE'H'EB STATES PATENT @FFEQE Edward W. Kellogg, Moorestown, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application May 24, 1935, Serial No. 23,167

30 Claims.

This invention relates to means for limiting curvature in sound records of the groove type, and has for its principal object the provision of an improved apparatus and method. of operation whereby excessive groove curvature of the record is prevented.

As pointed out in a copending application of Alda V. Bedford, Serial No. 8,292, filed February 26, 1935 and assigned to the same assignee as the present application, there is a Well recognized limit to the higher frequency amplitudes that may be satisfactorily reproduced. Among the factors which determine this limit is the radius of curvature of the tip of the reproducing stylus, which must be small relative to that of the sharpest curves in the record groove in order to prevent serious distortion. Recording at high general level is advantageous because it renders the background noise due to imperfections in the surface of the record, less conspicuous, but it may result in the recording of the strong high frequency peaks at such amplitudes that they will not be properly reproduced. Such peaks had better not be recorded at full strength, because the failure of the reproducing stylus to properly follow the groove causes r-asping and chattering sounds and results in excessive record wear.

One method of avoiding the recording of the strongest passages at such levels as to cause excessive groove curvature, while at the same time maintaining a high general level, is to employ an over-loading or saturating amplifier which cuts off the peaks of both high and low frequency signals. This, however, is not satisfactory for the reason that the limiting action of the amplifier Would act on many of the low frequency peaks, which do not need to be so limited. In accordance with the aforesaid application, this difficulty is avoided by (l) segregating the high and low frequency components, (2) passing the high frequencies through an amplifier adjusted to saturate and cut off excessive peaks, (3) passing the low frequencies through an amplifier which. does not cut off the peaks and (4) utilizing the combined outputs of the two amplifiers to actuate the record cutter.

The present invention differs from that disclosed by the aforesaid application in that the curvature limiting effect is confined to the particular waves Where the curvature would be excessive. For example, if a sound wave consists of components of several different frequencies, there are points at which the curvature due to all the components add. If the curvature is reduced only at such points, the unnecessary dis tortion resulting from uniform treatment of the frequency group is avoided.

In carrying out the invention, I may employ a wax cutter of the type at present in general use for making phonograph recordings, namely one in which the velocity of movement of the cutting stylus is proportional to the instantaneous value of the applied voltage, and I operate this cutter by means of an amplifier which modifies the voltage fed to the cutter in such a way that the rate of change of the voltage never exceeds a certain predetermined value. When such a cutter as I have described is used, the slope of the groove, or the tangent of the angle which the groove makes with the mean groove direction, is proportional at a given linear record speed, to the applied voltage, and the curvature of the groove is proportional to the time rate of change of this voltage. rate of change of the voltage supplied to the cutter, the voltage e1, which would usually be amplified and applied to the record cutter is first put through an amplifier stage which produces a voltage 62 proportional to d dt which is proportional to the final curvature. The voltage 62 is subjected to a modifying or limiting circuit. The resulting modified voltage ez is then applied to another amplifier stage which integrates it with respect to time giving The voltage 63 is then amplified and applied to the cutter. Voltage 63 is of the same wave shape as voltage 61 except for the limits imposed on the extreme rates of change as previously indicated.

The invention will be better understood from the following description. when considered in connection with the accompanying drawing, and its scope will be pointed out in the appended claims.

Referring to the drawing:

Fig. 1 illustrates the condition existing when the groove curvature of a lateral cut record becomes equal to that of the stylus. Although the groove Was cut with a finite curvature, the stylus tip must change its direction abruptly, or undergo infinite acceleration as indicated by the dotted line.

Fig. 2 illustrates the condition existing when the record curvature of a lateral cut record ex- For the purpose of limiting the s ceeds that of the stylus. Here it is quite impossible for the stylus to follow the path of the axis of the groove, and cutting away of the record material occurs immediately.

Fig. 3 is a wiring diagram of a four stage amplifier wherein a reactor in the plate circuit of the first stage converts the input voltage e1 into a voltage e2 which is proportional to rig a limiting device in the plate circuit of the second stage cuts off excessive peaks producing a modified voltage ez, a condenser in the plate circuit of the third stage integrates the voltage ez with respect to time, producing a voltage and the fourth stage amplifies the voltage es and applies it to the record cutter.

Fig. 4 illustrates an alternative connection for the plate circuit of the first tube of the amplifier illustrated by Fig. 3, but performing a similar difierentiation of the voltage e1.

Fig. 5 is an explanatory diagram relating to the operation of the current limiting device connected in the second stage plate circuit of the amplifier of Fig. 3.

Fig. 6 illustrates the stylus and groove relation required to avoid improper action of the stylus in the case of a hill and dale or vertical cut record.

Fig. '7 is an explanatory diagram relating to the operation of an amplifier suitable for producing the relation indicated by Fig. 6, and

Fig. 8 illustrates how the second stage of the amplifier of Fig. 3 may be modified to allow adjustment of the record curvature as the distance of the groove from the center of the record changes.

Fig. 1 illustrates a sound record groove l6 which has a curvature substantially equal to that of its cooperating stylus ll. Distortion of the reproduced wave occurs with sharp groove curvatures even before the condition illustrated in Fig. 1 is reached. When the curvature of groove becomes equal to that of the needle, or stylus tip, as shown in Fig. 1, there is established a limiting condition beyond which the stylus can not follow the groove and chattering and excessive record wear are produced. This condition is illustrated by Fig. 2 wherein the curvature of the groove l2 exceeds that of the stylus I l which fails to follow the groove and damages the record. This condition is much more harmful than that which would be produced by reducing the groove curvature sufliciently to permit the stylus to follow the groove. An amplifier adapted to produce this result is illustrated by Fig. 3.

The amplifier of Fig. 3 includes input terminals I 3 to which an audio frequency voltage suitable in wave form for operating a cutter of the type previously mentioned is applied, and output terminals i i through which the modified audio frequency voltages can be impressed on the operating coil of the record cutter. Between terminals l3 and M are interposed amplifier stages l5, I6, i! and i8. Connected in the plate circuit of the stage it is a reactor iii of such value that at the highest frequency of the sound spectrum to be recorded, its rea-ctance will be substantially lower than the plate resistance of the tube I5. The reactor ifi must also be so designed that its resistance is low compared with its reactance at the lowest frequency to be recorded. Under these conditions, the voltage at the plate of the tube will be proportional to the time rate of change of the voltage impressed on the input terminals I3. At any given record velocity, the groove curvature is proportional to the rate of change of the slope. The voltage at the plate of the first tube is proportional to the curvature of the groove to be cut. The blocking condensers and grid leaks shown in the various stages of the amplifier are assumed to have large enough capacities and resistances to transfer the plate alternating current voltages to the grids of the next stages with negligible phase shift and without appreciable change in magnitude.

For limiting the peaks of the amplified voltages a limiting device is connected in the plate circuit Of the tube l6. This limiting device includes a transformer 20 through which potential is applied to a full wave rectifier 2i of any suitable type, and a bias potential source 22 which ensures that no rectification takes place until the voltage developed in either half of the secondary of the transformer 28 exceeds the bias voltage. Voltages higher than this cause a loading of the plate of the tube it which prevents its swinging much further. Otherwise expressed, the tube l6 loaded with the rectifier 2! has a letter S characteristic such as that illustrated in Fig. 5 wherein the characteristic is indicated at A, the input wave at B and the output wave at C. A break in the characteristic is illustrated at D where the rectifier takes hold, and the resulting flattening of the tops of the output wave is shown in curve C. The fact that the rectifier tubes 2! have a higher resistance for very small currents than for higher currents, has the effect of rounding off the shoulder D in the characteristic curve A. The effect of the rectifier is to round off the tops of the waves which have high peaks, but it produces no distortion of waves whose peak voltages are less than the voltage of the biasing source 22. This limiting device is adjusted so as to operate only when the curvature of the resulting groove would, without the limiter, be greater than the predetermined allowable maximum. Various modifications of the limiting device are feasible. For example the tubes 25 might be replaced by crystal or copper oxide rectifier units, and the transformer 25! might be replaced by a choke or resistance feed and the limiting device operated through a blocking condenser. It would also be possible to obtain the desired limiting action by proper selection of the type of tube to be used at it and operating it under suitable conditions such as plate and. grid voltages, and cathode temperature. The arrangement shown in Fig. 3, however, is simple in principle and readily adjusted.

Having limited the voltage which is proportional to curvature. it is next necessary to apply this voltage in such a way as to again obtain a voltage suitable for operating the cutting device, namely a voltage proportional to the slope of the desired record groove. The conversion of voltage proportional to curvatin e to one proportional to the slope is a step in the opposite direction from that previously taken. One of the most satisfactory ways of accomplishing this is to load the tube H of the next stage with a condenser 23 so large that its reactance is small compared with the resistance of the plate circuit for all frequencies to be recorded. The voltage across this condenser is proportional to the time integral of the voltage impressed on the grid of the tube,

desired groove, and after a suitor to the slope of the further amplification may be applied to ably designed cutter.

The action of the loading reactor I9 is such as to cause an accentuation of the high frequency components in the same proportion that these contribute more to curvature than do low frequency components having the same voltage at the input end of the device but all components may contribute to produce maximum curvature in the record groove and they also contribute in identical manner to the production of maximum voltage at the limiting device. Were the movement of the cutter stylus controlled by inertia rather than mechanical resistance, the cutter coil could be supplied with current proportional to the voltage applied to the grid of tube IT. The limiting device could then operate to limit the extreme acceleration to which the cutter stylus is subjected. Since the cutters customarily used at the present time have their movements controlled by mechanical resistance rather than inertia reactance, this characteristic is taken care of by loading the third tube I! with a capacity 23. The reactance of this capacity is so small in comparison with the plate resistance of the tube that it does not appreciably influence the magnitude or wave shape of the alternating currents in the anode circuit, but these currents, which are identical in wave form with the voltage impressed on the grid of tube ll, result in a voltage across the terminal of the condenser 23, whose waveform is the integral of that of the'current. The action of the capacity accentuates the low frequencies and exactly neutralizesthe effect of the reactance l9 except insofar as the limiting device may have operated.

Assuming for illustration that a 2000and a 4000 cycle wave, one volt each, are impressed at the terminals l3, the 4000 cycle wave would produce on the grid of the third tube ll, twice the voltage produced by the 2000 cycle wave If the 2000 cycle wave produces one volt at the grid of the tube H, the4000 cycle wave produces two volts and the two waves together would produce three volts if they happen to add in phase. If the bias device 2i is set for less than 3 volts the'limiter would act on this combined peak. The condenser 23 which has twice the impedance at 2000 cycles that it has at 4.000 cycles functions to restore the original ratio of voltages of the high and low frequency components so that a wave shape suitable for operating a standard cutter is obtained. 1

.It should be understood that stylus acceleration is a measure of groove curvature. However, since the cutter is designed so that the stylus velocity (rather than acceleration) is proportional to current supplied, and velocity is the integral of acceleration, the correct relation is maintained by impressing the voltage which is proportional to acceleration on the grid of the tube ll, the capacity load on the plate circuit of this tube being utilized to provide an integration and thereby produce a voltage proportional to velocity, which is the integral of acceleration. This voltage is impressed on the grid of tube [8 and a current of identical wave form sent through the cutter coil. In practically all electromagnetic systems the force developed is proportional to the current. Therefore, the current through the actuating coil should be proportional to and in phase with the voltage applied to the grid of tube It, a condition which can in general be met by making the resistance of the cutter circuit high as compared with the inductive reactance of the coil, and by so designing any intervening amplifying or other apparatus that it introduces negligible distortion. On the other hand, it is also permissible to permit coil reactance to aiTect the current and to compensate for this by employing appropriate compensating circuits between tube I8 and the cutter.

The advantages of the curvature limiting system herein described are not limited to a cutter of the present mechanical resistance controlled type, but may be realized in connection with an inertia controlled cutter. Were it possible practically, to employ a cutter whose movements were controlled by inertia only, throughout the entire range of frequency to be recorded, the integrating operation performed by condenser 23 of Fig. 3 would not be required. With inertia control of cutter movements, the acceleration of the stylus will be proportional to the electromagnetic force developed, and since stylus acceleration is proportional to groove curvature, the force for operating the cutter (which I am assuming to be proportional to current) should be of the same waveshape as the voltage applied to the grid of tube IB. If, as would in general be the case, the inertia control .of the cutter holds over only a part of the required frequency range, compensation as for example by a loading condenser as shown in Fig. 3 would be needed, the compensation being effective over that part of the frequency range in which the stylus movements are controlled by mechanical resistance or factors other than inertia.

If a cutter is employed having resistance control through part of the frequency range and inertia control through some of the higher frequency range, the correct relation for proper operation of the cutter is maintained by employing a resistance 24 in series with a capacity 23. This resistance should be of such value that the capacity reactance and the resistance become equal at the same frequency that the mechanical resistance of the cutter becomes equal to its inertia reactance. Practically all cutters have of necessity a certain amount of elastic stifiness in the mounting of the movable parts. There will be a range of frequencies in the low frequency end of the sound spectrum, within which this stiffness will predominate over the mechanical resistance. When cutter movement is controlled by elasticity of the armature and stylus mounting (rather than by mechanical resistance or by inertia) the amplitude (instead of the slope or curvature) of the groove wave is proportional to the current supplied. The relation between amplitude and curvature corresponds to integrating twice. In order therefore to drive a stifiness controlled cutter, an additional stage of correction or integration of the same type employed between tubes ll and i8 of Fig. 3 would be required. Since the elastic control however, predominates over resistance through only a fraction of the frequency scale, in this extra stage the loading capacity (corresponding in circuit location to condenser 23) should have such a value that its reactance becomes equal to the resistance in series with it (corresponding to resistance 24) at the same frequency that the elastic reactance in the cutter becomes equal to its mechanical resistance. Since .the extra integration stage would have the efiect of raising the voltage of the low frequency components as compared with those of higher frequency, it may be desirable to offset this efiect by reducing the wise cause cutting over between adjacent grooves,

since the low frequency components while contributing little to groove curvature, contribute very largely to amplitude. If the cutter elastic reactance exceeds the mechanical resistance only at the very lowest frequenci.es,-the compensation or extra integration stage mentioned above may prove unnecessary.

It will be evident from the foregoing discussion that the application of my invention does not require that any particular design of cutter be employed, but a cutter of given characteristics having been chosen, electrical compensating circuits can be designed and applied between the tube I8 and the cutter, to give the desired overall characteristic. The desired over-all characteristic, for the purpose of invention, is such that the curvatures in the groove as cut will be proportional to the limited or modified voltage 6'2. Methods of compensating to obtain desired over-all characteristics are well known in the sound recording art, and examples have already been described herein, covering the cases which are most likely to arise. It is not even necessary that the record be of the present almost universally used type in which the slope of the groove is approximately proportional to the instantaneous pressures in the sound at the microphone. Were the reproducing devices to be employed of a different type, responding for example to deflection rather than velocity of the needle, a different form of record groove would be called for, but it would be desirable to limit extreme curvatures in exactly the same manner. The application of a curvature limiting device is however appropriate only in View of the finite mass of the reproducing stylus and the finite curvature of the stylus tip, which is an unavoidable feature in all mechanical reproduction.

A cutter of the design at present in wide use, in which several compliances and masses intervene between the armature and stylus constitutes a mechanical low pass filter. Such a cutter produces large phase shifts near the upper end of the frequency range and these phase shifts are very difficult to compensate. In order to avoid such diificulties, it is desirable to limit the maximum frequency supplied to the input terminals I 3 to a value well below that at which the mechanical filter properties of the cutter take effect. This can be accomplished by suitable filters interposed between the sound pick-up device and input terminals I3.

Fig. 4 shows a type of circuit which may be employed in place of the reactor IS between the first two stages of the amplifier of Fig. 3. In this circuit use is made or" the fact that the current through a condenser is proportional to the rate of change of the voltage impressed across the condenser. A condenser 25 is employed, having such small capacity that at no time does it, nor the resistance it, constitute an appreciable load on the plate of the tube I5. The voltage at the plate of tube i5 is th n equal to a constant times the voltage impressed on the grid of tube I 5. The resistance 26 is also low in comparison with reactance of condenser 25, so that throughout the essential frequency range the current flowing through condenser 25 and resistance 26 is equal to the voltage at the plate of tube I5 divided by the capacity reactance. The drop across the resistance which is proportional to this condenser charging current, which is proportional to the rate of change of the impressed voltage. This resistance drop may be impressed on the grid of the following tube I6.

The amplifier tubes I5, I6, I! and I8are not strictly essential to carrying out the differentiation, limiting and integration functions of the circuits of Figs. 3 and 4, but serve in a valuable way to isolate the several special circuits, so they will not react on each other, and also to make up for the losses of voltage which would otherwise occur.

In the case of hill and dale records, there are also limits to the curvatures which the reproducing stylus can follow, but here the problem assumes somewhat different aspects. Fig. 6 shows the shape that would have to be given the groove bottom in order to cause the center of the stylus tip to follow a sinusoidal path, indicated by the dotted line. As will be seen from Fig. 6, it is desirable to cut the groove of a hill and dale type of record with sharp hills and long Valleys to facilitate tracking of the stylus. In order to limit the curvature only in one direction or in valleys of the record, the amplifier should have a characteristic similar to that illustrated in Fig. '7 wherein E represents the tube characteristic, F the input voltage wave, and G the output voltage wave. Such a characteristic may be obtained with the amplifier of Fig. 3 by omitting the limiting device and biasing the grid of tube I! so that it operates on a strongly curved characteristic. An alternative arrangement for producing the same result is to employ a single anode rectifier with suitable bias potential in place of the double acting rectifier of Fig. 3. Such a rectifier may for example be utilized to impose a heavy load on the positive swings and a smaller load for the negative swings.

Since the linear speed of the record becomes less as the center is approached, the recorded waves are shorter and consequently the curvatures are increased. It is therefore desirable to change the curvature controlling or limiting characteristic as the record progresses. This can be readily accomplished in the case of the limiting device of Fig. 3 by continuously decreasing the Voltage of the biasing device 22, but I prefer toaccomplish the same result by modifying the connections of stages I6 and IT as indicated by Fig. 8 which shows two gain controls 2'! and 28, one preceding and the other following the tube I6 and both operated by single control member 3|, so that there is a fixed relation between the positions of the two. As the center of the record is approached, the first control is turned up and the second control turned down to exactly equalize the over-all gain, but to increase the voltage applied to the limiter or curvature controlling device, thereby causing it to operate at continuously lower cutting tool amplitudes. The type of apparatus illustrated in Fig. 8 can'be applied to the curvature controlling system for either lateral or hill and dale records.

Having thus described my invention, I claim:

1. A sound recorder including a multi-stage amplifier, means connected in the plate circuit of a leading stage of said amplifier for converting acoustically modulated voltage to a voltage proportional to the rate of change of said acoustically modulated voltage, and means connected in a following stage of said amplifier for suppressing excessive peaks in said converted voltage.

2. In combination with a sound recorder, a multi-stage amplifier, means connected in the plate circuit of a leading stage of said amplifier for converting acoustically modulated voltage to a voltage proportional to the record groove curvature, and a biased rectifier connected in the plate circuit of a following stage for suppressing excessive peaks of said converted voltage. 3. A sound recording system including a multistage amplifier, means connected in the plate circuit of a leading stage of said amplifier for converting acoustically modulated voltage to a voltage substantially proportional to the record groove curvature, means connected in an intermediate stage of said amplifier for suppressing excessive peaks of said converted voltage, and means connected in a following stage of said amplifier for integrating said peak-suppressed voltage to a wave form similar to that of said acoustically modulated voltage except for the suppression of said peaks.

4. A sound recording system including a sequence of electrical circuits, means connected in a leading stage or" said system for converting acoustically modulated voltage to a voltage proportional to the rate of change of said acoustically modulated voltage, means connected in an intermediate stage of said system for modifying peaks of said converted voltage, and means con nected in a following stage of said system for integrating said modified voltage to a wave form similar to that of said acoustically modulated voltage except for the modification of said peaks.

5. A sound recording system including a multistage amplifier, means connected in the plate circuit of a leading stage of said amplifier for converting acoustically modulated voltage to a voltage proportional to the rate of change of said acoustically modulated voltage, means connected in an intermediate stage of said amplifier for asymmetrically modifying said converted voltage, and means connected in a following stage of said amplifier for integrating said modified voltage to a wave form similar to that of said acoustically modulated voltage except for the action of the said modifying means.

6. A sound recording system including a multi- .stage amplifier, means connected in the plate circuit of a leading stage of said amplifier for converting acoustically modulated voltage to a voltage proportional to the record groove curvature, means for suppressing peaks in said converted voltage, and means operable to vary the effect of said peak suppressing means without changing the gain of said amplifier.

'l. A sound recording system including a multistage amplifier, means connected in the plate circuit of a leading stage of said amplifier for converting acoustically modulated voltage to a voltage proportional to the rate of change of said acoustically modulated voltage, means for modifying the wave form of said converted voltage, and means operable to vary the effect of said wave form modifying means: without changing the gain of said amplifier.

8. The method of recording an acoustic-ally modulated voltage which includes converting said voltage to a voltage proportional to the curvature of the record groove, modifying excessive peaks of said converted voltage, integrating said peak modified voltage whereby a voltage is obtained similar to that of said acoustically modulated voltage except for the modification of said peaks,

and varying the degree of modification of said peaks.

9. The method of recording an acoustically modulated voltage by means of an electrically actuated record cutter, which includes converting said voltage to a voltage substantially proportional to the curvature of the desired record groove, asymmetrically modifying peaks of said converted voltage, integrating said modified voltage whereby a voltage is obtained suitable for operating said record cutter, and varying the degree of asymmetrical modification of said peaks.

10. The method of recording an acoustically modulated voltage by means of an electrically actuated record cutter, which includes converting said voltage to a voltage substantially proportional to the curvature of the desired record groove, suppressing excessive peaks of said converted voltage, integrating said peak suppressed voltage whereby a voltage is obtained suitable for operating said record cutter, and varying the suppression of said peaks.

11. A sound recording system including means for converting acoustically modulated voltage to a voltage proportional to the rate of change of said acoustically modulated voltage, and means having non-linear characteristics for modifying said converted voltage.

12. A sound recorder including means for converting acoustically modulated voltage to a voltage proportional to the record groove curvature, means for modifying the peaks of said converted voltage, and means for integrating said modified voltage to a wave form similar to that of said acoustically modulated voltage except for the modification of said peaks.

13. A sound recorder including means for con-- verting acoustically modulated voltage to a voltage proportional to the record groove curvature, means for suppressing the peaks of said converted voltage, and means for integrating said peak suppressed voltage to a wave form similar to that of said acoustically modulated voltage except for the suppression of said peaks.

14. The method of amplifying an alternating voltage to be recorded which includes converting said voltage to a voltage proportional to the rec- 0rd groove curvature, suppressing excessive peaks of said converted voltage, and integrating said peak suppressed voltage to a wave shape which is similar to that of said alternating current voltage except for the suppression of said peaks.

15. The method of recording acoustically modulated voltage which includes converting said voltage to a voltage proportional to the record groove curvature, suppressing excessive peaks of said converted voltage, integrating said peak suppressed voltage to a wave form which is similar to that of said acoustically modulated voltage except. for the suppression of said peaks.

16. In an impulse recorder, a multi-stage amplifier, reactance means connected with a leading stage of said amplifier for converting the impulses to be recorded to impulses substantially proportional to the record curvature, and a nonlinear circuit connected with an intermediate stage of said amplifier for modifying the wave shape of said converted impulses.

17. In an impulse recorder, a multi-stage amplifier, reactance means connected with a leading stage of said amplifier for converting the impulses to be recorded to impulses substantially proportional to the record curvature, and a biased rectifier connected with an intermediate stage of said amplifier for suppressing excessive peaks of said converted impulses.

18. An impulse recorder including a multistage amplifier and a record cutting device, a reactance connected with a leading stage of said amplifier for converting the impulses to be recorded to impulses substantially proportional to the record groove curvature, a. biased rectifier connected with an intermediate stage of saidamplifier for suppressing peaks of said converted impulses, and reactance means connected with .a following stage of said amplifier for integrating said peak suppressed impulses to a wave form suitable for operating said record cutting device.

19. An impulse recorder including a multi-stage amplifier, and a record cutting device, reactance means connected with a leading stage of said amplifier for converting the impulses to be recorded to impulses substantially proportional to the record groove curvature, a biased rectifier connected with an intermediate stage of said amplifier for suppressing peaks of said converted impulses, and means operable independently of the gain of said amplifier for adjusting the effect of said rectifier on said peaks.

20. In a sound recording system means for producing a voltage proportional to the curvature of the desired record groove, means for modifying the wave form of said voltage, and means including an electrical compensating circuit for causing a recording stylus to undergo acceleration proportional to said modified voltage.

21. In a sound recording system means for producing a voltage proportional to the curvature of r the desired record groove, means for modifying the wave form of said voltage, means including an electrical compensating circuit for causing a recording stylus to undergo acceleration proportional to said modified voltage, and means related to the position of the cutting device relative to the center of the record, for altering the degree of action of said Wave form modifying means.

22. A sound recording system including means for converting acoustically modulated voltage to a voltage substantially proportional to the record groove curvature, means having non-linear characteristics for suppressing peaks of said converted voltage, and means responsive to said peak suppressed voltage for cutting a record groove whose slope is proportional to the integral of said peak suppressed voltage.

23. A sound recording system including means for converting acoustically modulated voltage to a voltage substantially proportional to the record groove curvature, means having non-linear characteristics for modifying said converted voltage, and means responsive to said modified voltage for cutting a record groove whose slope is proportional to the integral of said modified voltage.

24. In a sound recording system for hill and dale records, means for producing a voltage substantially proportional to the curvature of the desired record groove, means including an electrical circuit having non-linear characteristics for modifying said voltage whereby the peaks of one polarity are increased relative to those of the opposite polarity, and groove cutting means responsive to said modified voltage having such characteristics that the vibratory velocity of the cutting stylus is proportional to the time integral of said modified voltage.

25. In a sound recording system for hill and dale records, means for producing a voltage substantially proportional to the curvature of the desired record groove, means including an electrical circuit having non-linear characteristics for modifying said voltage whereby the peaks of one polarity are reduced relative to those of the opposite polarity, and groove cutting means responsive to said modified voltage having such characteristics that the vibratory velocity of the cutting stylus is proportional to the time integral of said modified voltage, and means for changing the effect of said modifying circuit whereby it produces greater wave shape alterations when the cutting device is operating near the center of the record.

26. In a sound recording system for hill and dale records, means for producing a voltage substantially proportional to the curvature of the desired record groove, means including an electrical circuit having non-linear characteristics for modifying said voltage whereby the peaks of one polarity are reduced relative to those of the opposite polarity, groove cutting means responsive to said modified voltage having such characteristics that the vibratory velocity of the cutting stylus is proportional to the time integral of said modified voltage, and means for changing the effect of said modifying circuit whereby the smaller the radius of the groove being cut the greater is the effect of said non-linear circuit in modifying the wave shape.

27. The method of modifying an alternating voltage to be recorded which includes production of a second voltage proportional to the rate of change of said original voltage and suppressing in said second voltage peaks of one polarity relative to the peaks of the opposite polarity.

28. A vertically out sound record in which the curvature at the bottoms of the valleys have been decreased independently of the record material resistance and relative to the curvatures at the tops of the hills.

29. The method of recording sound which comprises producing a voltage substantially proportional to the rate of change of pressure in the sound waves to be recorded, causing a non-linear distortion of said voltage, and cutting a record groove having a slope proportional to the instantaneous value of the time integral of said distorted voltage.

30. The method of cutting a sound record groove which comprises the production of a voltage substantially proportional to the rate of change of pressure in the sound to be recorded, causing a non-linear distortion of said voltage wave, causing the degree of said distortion to vary in accordance with the radius of the groove being cut, and causing the cutting stylus to undergo accelerations proportional to the instantaneous values of said distorted voltage.

EDWARD W. KELLOGG. 

